Armenite

Armenite
	Armenite from Isenwegg, Wasenalp, Ganter Valley, Brig , Valais , Switzerland
General and classification
other names	Calciocelsian
chemical formula	BaCa 2 Al 3 [Al 3 Si 9 O 30 ] • 2H 2 O
Mineral class; (and possibly department)	Silicates and Germanates
System no. to Strunz ; and to Dana	9.CM.05 ( 8th edition : VIII / E.22) ; 63.02.01b.01
Similar minerals	Milarite, apatite, quartz, colorless beryl
Crystallographic Data
Crystal system	orthorhombic
Crystal class ; symbol	orthorhombic-dipyramidal; 2 / m 2 / m 2 / m
Space group	Pnna (No. 52)
Lattice parameters	a = 13.874 (2) Å ; b = 18.660 (2) Å; c = 10.697 (1) Å ; α = 90 °; β = 90 °; γ = 90 °
Formula units	Z = 4
Frequent crystal faces	{100}, {001}, {112}, {110}, {102}
Twinning	Penetrating Twins
Physical Properties
Mohs hardness	7 to 8
Density (g / cm 3 )	2.76
Cleavage	good to completely parallel to the pseudo-hexagonal prism surfaces
colour	colorless, green
Line color	White
transparency	translucent to cloudy
shine	Glass gloss
Crystal optics
Refractive indices	n α = 1.551 (2) ; n β = 1.559 (2) ; n γ = 1.562 (2)
Birefringence	δ = 0.008 to 0.011
Optical character	negative
Axis angle	2V = 60 ° (2)
Pleochroism	-
Other properties
Chemical behavior	drains from ≈ 500 ° C

The mineral armenite (Calciocelsian) is a rare ring silicate from the milarite group and has the chemical composition Ba _0.991 K _0.029 Na _0.082 Ca _2.016 Al _5.864 Si _9.054 O ₃₀ · 2H ₂ O.

Armenite crystallizes with orthorhombic symmetry and forms pseudo-hexagonal, prismatic crystals . Armenite is usually colorless or pale green with a glass-like gloss and good cleavage parallel to the prismatic surfaces. With a Mohs hardness of 7 to 8, armenite is slightly harder than quartz.

Etymology and history

The description as an independent mineral was made in 1939 by Henrich Neuman at the University of Oslo in Norway , after the sample had been collected since 1877 by mineralogy student OA Corneliussen in the poor mine near Kongsberg and named " Epidote ?" labeled, in the collection of the Oslo Institute for Mineral Orgy. The name Armenit goes back to the place where it was found, the poor mine.

In another publication two years later, Neuman showed the relationship between armenite and milarite and questioned the classification as zeolite - the dewatering temperature of 500 to 600 ° C is too high for zeolitic water.

classification

In the outdated but still partially in use 8th edition of the mineral classification by Strunz of Armenit belongs to the general ward of the " Ring silicates (cyclo silicates)" where he collaborated with Almarudit , Berezanskit , Brannockit , Chayesit , Darapiosit , Dusmatovit , Eifelit , Emeleusit , Faizievit , Merrihueit , Oftedalit , Osumilith , Osumilith- (Mg) , Poudretteit , Roedderite , Shibkovite , Sogdianite , Sugilite , Trattnerite , Yagiit and Yakovenchukit- (Y) the " Milarite-Osumilite-Group " with the system no. VIII / E.22 forms.

The 9th edition of Strunz's mineral systematics, which has been in force since 2001 and is used by the International Mineralogical Association (IMA), also classifies armenite in the "ring silicates" section. This is, however, further subdivided according to the structure of the rings, so that the mineral can be found in the sub-section "[Si ₆ O ₁₈ ] ¹²⁻ -six double rings" according to its structure . With Almarudit, Berezanskit, Brannockit, Chayesit, Darapiosit, Dusmatovit, Eifelit, Friedrichbeckeit , Klöchit , Merrihueit, Milarit, Oftedalit, Osumilith, Osumilith- (Mg), Poudretteit, Roedderit, Shibkovite, Yagiitite, Sugagiite belong to the Sogdianite " Milaritgruppe " with the system no. 9.CM.05 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the armenite to the class of "silicates and germanates", but there in the more finely divided division of "ring silicates: condensed rings". Here he is the only member of the " Milarit-Osumilitgruppe " with the system no. 63.02.01b can be found in the subsection " Ring Silicates : Condensed, 6-membered Rings".

Crystal structure

Armenite crystallizes in the structure of milarite . The distribution of the aluminum in the silicate double rings is completely ordered. The 9-fold coordinated B position is completely occupied by H ₂ O, which is also installed in an orderly manner on a sub-position of the split B position. The oxygen ion of the H ₂ O molecule increases the coordination of the neighboring Ca ion on the A position to 7, i.e. i.e., it also forms a weak bond with Ca. These three factors, Al-Si order, H ₂ O on the B 'subposition and increase in the Ca coordination, lead to a lowering of the symmetry of armenite from hexagonal to orthorhombic with the space group

Pnna (No. 52) and the lattice parameters a = 13.874 (2) Å , b = 18.660 (2) Å and c = 10.697 (1) ÅTemplate: room group / 52

or Pnc 2 (No. 30) and the lattice parameters a = 18,660 (2) Å, b = 10,697 (1) Å and c = 13,874 (2) ÅTemplate: room group / 30

with 4 formula units per unit cell .

Education and Locations

Armenite forms in barium- rich environments at low pressure and temperatures below 400 ° C. These are mostly late magmatic hydrothermal, sulphidic mineralizations such as the type locality near Kongsberg, Norway or sulphide-containing rocks modified by hydrothermal solutions. An occurrence of armenite associated with gneiss has been documented from the Broken Hill deposit , New South Wales , Australia , which suggests significantly higher formation temperatures. The eye gneiss that occurs on the Wasenalp north of the Wasenhorn in the Simplon region near Brig , Valais , Switzerland , carries various rock-forming barium minerals and contains armenite crystals up to 2.5 cm in size in cavities in alpine fissures, which are considered the world's largest and best representatives of this type of mineral.

Individual evidence

^ ^A ^b Hans Anton Stalder, Albert Wagner, Stefan Graeser, Peter Stuker: Mineralienlexikon der Schweiz . 1st edition. Wepf & Co., Basel 1998, ISBN 3-85977-200-7 , p. 50 .
↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p H. Neumann (1941): Armenite, a water-bearing barium-calcium-alumosilicate , In: Norsk Geologisk Tidsskrift , 21, pp. 19-24 ( PDF, 683 kB )
↑ ^a ^b H. Neumann (1939): Armenite, a new mineral , In: Norsk Geologisk Tidsskrift , 19, pp. 312–313 ( PDF, 55 kB )
↑ T. Armbruster, M. Czank (1992): H ₂ O ordering and superstructures in armenite, BaCa ₂ AlSi ₉ O ₃₀ · 2H ₂ O: A single-crystal X-ray and TEM study , In: American Mineralogist , Volume 77 , Pp. 422–430 ( PDF, 2.0MB )
↑ T. Armbruster (1999): Si, Al ordering in the double-ring silicate armenite, BaCa2Al6Si9O30 · 2H2O: A single-crystal X-ray and 29Si MAS NMR study , In: American Mineralogist , Volume 84, pp. 92-101 ( PDF, 338kB )
↑ FC Hawthorne, M. Kimata, P. Černý, N. Ball, GR Rossman, JD Grice (1991): The crystal chemistry of the milarite-group minerals , In: American Mineralogist , Volume 76, pp 1836-1856 ( PDF , 2.6MB )

[StalderWagnerGraeserStuker-1] A ^b Hans Anton Stalder, Albert Wagner, Stefan Graeser, Peter Stuker: Mineralienlexikon der Schweiz . 1st edition. Wepf & Co., Basel 1998, ISBN 3-85977-200-7 , p. 50 .

[Neumann_1941-2] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p H. Neumann (1941): Armenite, a water-bearing barium-calcium-alumosilicate , In: Norsk Geologisk Tidsskrift , 21, pp. 19-24 ( PDF, 683 kB )

[Neumann_1939-3] H. Neumann (1939): Armenite, a new mineral , In: Norsk Geologisk Tidsskrift , 19, pp. 312–313 ( PDF, 55 kB )

[Armbruster_Czank_1992-4] T. Armbruster, M. Czank (1992): H ₂ O ordering and superstructures in armenite, BaCa ₂ AlSi ₉ O ₃₀ · 2H ₂ O: A single-crystal X-ray and TEM study , In: American Mineralogist , Volume 77 , Pp. 422–430 ( PDF, 2.0MB )

[Armbruster_1999-5] T. Armbruster (1999): Si, Al ordering in the double-ring silicate armenite, BaCa2Al6Si9O30 · 2H2O: A single-crystal X-ray and 29Si MAS NMR study , In: American Mineralogist , Volume 84, pp. 92-101 ( PDF, 338kB )

[Hawthorne_et_al._1991-6] FC Hawthorne, M. Kimata, P. Černý, N. Ball, GR Rossman, JD Grice (1991): The crystal chemistry of the milarite-group minerals , In: American Mineralogist , Volume 76, pp 1836-1856 ( PDF , 2.6MB )